# Linear representation theory of semidihedral group:SD16

View linear representation theory of particular groups | View other specific information about semidihedral group:SD16

## Summary

We shall use the semidihedral group of order 16 with the following presentation:

$\langle a,x \mid a^8 = x^2 = e, xax^{-1} = a^3 \rangle$.

Item Value
degrees of irreducible representations over a splitting field (e.g., $\mathbb{C}$ or $\overline{\mathbb{Q}}$) 1,1,1,1,2,2,2 (1 occurs 4 times, 2 occurs 3 times)
maximum: 2, lcm: 2, number: 7, sum of squares: 16
Schur index values of irreducible representations over a splitting field 1,1,1,1,1,1,1
smallest ring of realization (characteristic zero) $\mathbb{Z}[\sqrt{-2}]$ = $\mathbb{Z}[\sqrt{2}i]$ = $\mathbb{Z}[t]/(t^2 + 2)$
Same as ring generated by character values.
minimal splitting field, i.e., smallest field of realization (characteristic zero) $\mathbb{Q}(\sqrt{-2})$ = $\mathbb{Q}(\sqrt{2}i)$ = $\mathbb{Q}[t]/(t^2 + 2)$
Same as field generated by character values, because all Schur index values are 1.
See minimal splitting field need not be cyclotomic
condition for a field to be a splitting field The characteristic should not be equal to 2, and the polynomial $t^2 + 2$ must split.
For a finite field of size $q$, this is equivalent to saying that $q \equiv 1 \pmod 8$ or $q \equiv 3 \pmod 8$.
minimal splitting field (characteristic $p \ne 0, 2$) Case $p \equiv 1 \pmod 8$ or $p \equiv 3 \pmod 8$: prime field $\mathbb{F}_p$
Case $p \equiv 5 \pmod 8$ or $p \equiv 7 \pmod 8$: Field $\mathbb{F}_{p^2}$, quadratic extension of prime field.
smallest size splitting field Field:F3.
degrees of irreducible representations over the rational numbers 1,1,1,1,2,4

## Representations

### Summary information

Below is summary information on irreducible representations that are absolutely irreducible, i.e., they remain irreducible in any bigger field, and in particular are irreducible in a splitting field. We assume that the characteristic of the field is not 2, except in the last column, where we consider what happens in characteristic 2.

Name of representation type Number of representations of this type Degree Schur index Criterion for field Kernel (a normal subgroup of semidihedral group:SD16 -- see subgroup structure of semidihedral group:SD16) Quotient by kernel (on which it descends to a faithful representation) Characteristic 2
trivial 1 1 1 any whole group trivial group works
sign representation with kernel $\langle a \rangle$ 1 1 1 any Z8 in SD16: $\langle a \rangle$ cyclic group:Z2 works, same as trivial
sign representation with kernel a maximal dihedral subgroup 1 1 1 any D8 in SD16: $\langle a^2, x \rangle$ cyclic group:Z2 works, same as trivial
sign representation with kernel a maximal quaternion subgroup 1 1 1 any Q8 in SD16: $\langle a^2, ax \rangle$ cyclic group:Z2 works, same as trivial
two-dimensional irreducible, not faithful 1 2 1 any center of semidihedral group:SD16: $\langle a^4 \rangle$ dihedral group:D8 indecomposable but not irreducible
two-dimensional faithful irreducible 2 2 1 The polynomial $t^2 + 2$ must split, i.e., $-2$ must have a square root trivial subgroup, i.e., it is a faithful linear representation semidihedral group:SD16  ?

Below are representations that are irreducible over a non-splitting field, but split over a splitting field.

Name of representation type Number of representations of this type Degree Criterion for field What happens over a splitting field? Kernel Quotient by kernel (on which it descends to a faithful representation)
four-dimensional faithful irreducible 1 4 The polynomial $t^2 + 2$ must not split, i.e., $-2$ must not have a square root splits into the two two-dimensional faithful irreducibles. trivial subgroup, i.e., it is a faithful linear representation semidihedral group:SD16

### Trivial representation

The trivial representation or principal representation (whose character is called the trivial character or principal character) sends all elements of the group to the $1 \times 1$ matrix $(1)$:

Element Matrix Characteristic polynomial Minimal polynomial Trace, character value
$e$ $(1)$ $t - 1$ $t - 1$ 1
$a$ $(1)$ $t - 1$ $t - 1$ 1
$a^2$ $(1)$ $t - 1$ $t - 1$ 1
$a^3$ $(1)$ $t - 1$ $t - 1$ 1
$a^4$ $(1)$ $t - 1$ $t - 1$ 1
$a^5$ $(1)$ $t - 1$ $t - 1$ 1
$a^6$ $(1)$ $t - 1$ $t - 1$ 1
$a^7$ $(1)$ $t - 1$ $t - 1$ 1
$x$ $(1)$ $t - 1$ $t - 1$ 1
$ax$ $(1)$ $t - 1$ $t - 1$ 1
$a^2x$ $(1)$ $t - 1$ $t - 1$ 1
$a^3x$ $(1)$ $t - 1$ $t - 1$ 1
$a^4x$ $(1)$ $t - 1$ $t - 1$ 1
$a^5x$ $(1)$ $t - 1$ $t - 1$ 1
$a^6x$ $(1)$ $t - 1$ $t - 1$ 1
$a^7x$ $(1)$ $t - 1$ $t - 1$ 1

### Sign representation with kernel $\langle a \rangle$

This representation is a one-dimensional representation sending everything in the cyclic subgroup $\langle a \rangle$ (see Z8 in SD16) to $(1)$ and everything outside it to $(-1)$.

To keep the description short, we club together the cosets rather than having one row per element:

Elements Matrix Characteristic polynomial Minimal polynomial Trace, character value
$\{ e,a,a^2,a^3,a^4,a^5,a^6,a^7 \}$ $(1)$ $t - 1$ $t - 1$ 1
$\{ x,ax,a^2x,a^3x,a^4x,a^5x,a^6x,a^7x \}$ $(-1)$ $t + 1$ $t + 1$ -1

### Sign representation with kernel $\langle a^2, x \rangle$

There is a sign representation with kernel $\langle a^2, x\rangle$ which is dihedral group:D8 (see D8 in SD16). Everything inside the subgroup goes to $(1)$ and everything outside the subgroup goes to $(-1)$.

To keep the descriptions short, we club together the cosets rather than having one row per element:

Elements Matrix Characteristic polynomial Minimal polynomial Trace, character value
$\{ e,a^2,a^4,a^6,x,a^2x,a^4x,a^6x \}$ $(1)$ $t - 1$ $t - 1$ 1
$\{ a,a^3,a^5,a^7,ax,a^3x,a^5x,a^7x \}$ $(-1)$ $t + 1$ $t + 1$ -1

### Sign representation with kernel $\langle a^2, ax \rangle$

There is a sign representation with kernel $\langle a^2, ax\rangle$ which is quaternion group (see Q8 in SD16). Everything inside the subgroup goes to $(1)$ and everything outside the subgroup goes to $(-1)$.

To keep the descriptions short, we club together the cosets rather than having one row per element:

Elements Matrix Characteristic polynomial Minimal polynomial Trace, character value
$\{ e,a^2,a^4,a^6,ax,a^3x,a^5x,a^7x \}$ $(1)$ $t - 1$ $t - 1$ 1
$\{ a,a^3,a^5,a^7,x,a^2x,a^4x,a^6x \}$ $(-1)$ $t + 1$ $t + 1$ -1

### Two-dimensional irreducible unfaithful representation

This representation has kernel equal to $\langle a^4 \rangle$ -- center of semidihedral group:SD16. It descends to a faithful irreducible two-dimensional representation of the quotient group, which is isomorphic to dihedral group:D8.

To keep the descriptions short, we club together the cosets rather than having one row per element:

Element Matrix Characteristic polynomial Minimal polynomial Trace, character value Determinant
$\{ e, a^4 \}$ $\begin{pmatrix} 1 & 0 \\ 0 & 1 \\\end{pmatrix}$ $(t - 1)^2 = t^2 - 2t + 1$ $t - 1$ 2 1
$\{ a, a^5 \}$ $\begin{pmatrix}0 & -1 \\ 1 & 0 \\\end{pmatrix}$ $t^2 + 1$ $t^2 + 1$ 0 1
$\{ a^2, a^6 \}$ $\begin{pmatrix} -1 & 0 \\ 0 & -1 \\\end{pmatrix}$ $(t + 1)^2 = t^2 + 2t + 1$ $t + 1$ -2 1
$\{ a^3, a^7 \}$ $\begin{pmatrix}0 & 1 \\ -1 & 0 \\\end{pmatrix}$ $t^2 + 1$ $t^2 + 1$ 0 1
$\{ x, a^4x \}$ $\begin{pmatrix}1 & 0 \\ 0 & -1 \\\end{pmatrix}$ $t^2 - 1$ $t^2 - 1$ 0 -1
$\{ ax, a^5x \}$ $\begin{pmatrix}0 & 1 \\ 1 & 0 \\\end{pmatrix}$ $t^2 - 1$ $t^2 - 1$ 0 -1
$\{ a^2x, a^6x \}$ $\begin{pmatrix}-1 & 0 \\ 0 & 1 \\\end{pmatrix}$ $t^2 - 1$ $t^2 - 1$ 0 -1
$\{ a^3x, a^7x \}$ $\begin{pmatrix}0 & -1 \\ -1 & 0 \\\end{pmatrix}$ $t^2 - 1$ $t^2 - 1$ 0 -1

## Character table

FACTS TO CHECK AGAINST (for characters of irreducible linear representations over a splitting field):
Orthogonality relations: Character orthogonality theorem | Column orthogonality theorem
Separation results (basically says rows independent, columns independent): Splitting implies characters form a basis for space of class functions|Character determines representation in characteristic zero
Numerical facts: Characters are cyclotomic integers | Size-degree-weighted characters are algebraic integers
Character value facts: Irreducible character of degree greater than one takes value zero on some conjugacy class| Conjugacy class of more than average size has character value zero for some irreducible character | Zero-or-scalar lemma

Below is the character table over a splitting field, where $\sqrt{-2}$ stands for any chosen square root of $-2$:

Representation/conjugacy class representative $\{ e \}$ (size 1) $\{ a^4 \}$ (size 1) $\{ a^2, a^6 \}$ (size 2) $\{ a, a^3 \}$ (size 2) $\{ a^5, a^7 \}$ (size 2) $\{ x, a^2x, a^4x, a^6x \}$ (size 4) $\{ ax, a^3x, a^5x, a^7x \}$ (size 4)
$\langle a \rangle$-kernel sign 1 1 1 1 1 -1 -1
$\langle a^2, x \rangle$-kernel sign 1 1 1 -1 -1 1 -1
$\langle a^2, ax \rangle$-kernel sign 1 1 1 -1 -1 -1 1
two-dimensional unfaithful, kernel is center 2 2 -2 0 0 0 0
first faithful irreducible representation 2 -2 0 $\sqrt{-2}$ $-\sqrt{-2}$ 0 0
second faithful irreducible representation 2 -2 0 $-\sqrt{-2}$ $\sqrt{-2}$ 0 0